I'm looking into having a metal garage built. Probably going to be 24 x 30ish. I'm worried about the building having condensation inside of it. I had a guy tell me that if I put plastic under the concrete area before having it poured that it'll stop all the condensation. Is there any truth to that?

How much does adding insulation to the top actually help these stand alone buildings? It'll get full sun pretty much all day until around 4-5 in the summer then it'll get a little shade. Any color buildings better for not getting as hot? It'll basically hold tools and toys. It'll have electric and water but not be used for anything where I'll need AC.